1. Field of the Invention
The present invention relates generally to air cooled turbine airfoils, and more specifically to turbine airfoils with micro cooling channels.
2. Description of the Related Art including information disclosed under 37 CFR 1.97 and 1.98
In early engines, the power and performance of the gas turbine engines were limited by the material characteristics on operating temperature. To allow for higher turbine inlet temperatures, cooling were used in the early stage airfoils to allow for gas flow temperatures to actually exceed the material thermal limits. The first generation of air cooled turbine airfoils included radial passages extending from a supply channel formed within the blade root and through the airfoil portion to eventually discharge out the blade tip. These passages were straight and produced convection cooling only.
The next generation of air cooled airfoils included impingement cooling along with the convection cooling of the internal metal structure of the airfoil. Improved compressor compression ratios allowed for the use of higher cooling air pressures. Impingement cooling would direct a jet of pressurized cooling air onto the inner wall surface that was exposed to heat from the high temperature gas flow, which is referred to as backside cooling.
The next and latest generation of air cooled airfoils included film cooling of the external airfoil surface. Film cooling holes located at the highest external airfoil temperatures would discharge jets of air that would develop a layer of cooling air to blanket the metal surface from the hot gas flow over the airfoil. Elaborate designs for the film cooling holes have evolved into film holes that provide wider and longer lasting film layers.
Air cooled turbine airfoils are produced using the well known investment casting process in which a core having the shape of the desired internal cooling circuitry for the airfoil would be covered with a wax material to form a pattern of the airfoil. An outer ceramic coating would be applied over the wax pattern to form a mold for the inner and outer surfaces of the airfoil. The wax pattern would be leached away to leave the core and the outer airfoil surface in the mold. Molten metallic alloys material would then be poured into the mold to solidify over the core to produce the detailed internal cooling circuitry of the airfoil. The ceramic core material would then be leached away from the solidified metallic airfoil to leave the finished airfoil having the outer airfoil shape and the internal cooling circuitry. Film cooling holes would then be drilled into the airfoil walls to produce the finished airfoil. FIGS. 1 and 2 show a prior art air cooled airfoil with a multiple impingement trailing edge cooling design.
One major problem with the investment casting process used to produce a modern air cooled turbine airfoil is that the defect rate of airfoils is very high due to the ceramic cores being broken during or after the casting process to create the core, or during the casting process when the molten metallic material flows around the core details. The cores are made from a ceramic material which is very brittle. Also, the size of the features that the core will reproduce is limited to around 0.010 inches (0.25 mm). in other words, the size of small cooling air passages formed within the airfoil using a ceramic core is limited to no smaller than 0.010 inches because of the ceramic material properties. Ceramic cores are limited in size due to the granular structure of the material. Smaller sizes are not capable of being produced that can create the smaller cooling features under the 0.010 inches.
On of the major advances in gas turbine engine design of late has been the use of advanced computational fluid dynamics (CFD) modeling. With CFD modeling of the airfoil cooling circuitry, optimized designs have been discovered to improved film cooling parameters such as the blowing ratio, injection angle, and discharge coefficient and discharge trajectory. However, air cooled turbine airfoils using these CFD optimized designs cannot be produced using the current investment casting process because of the minimum core size.